Thin Slab Casting

Abstract:

Thin Slab Casting is a significant evolutionary step in casting methodology and allowed much more efficiency and therefore cost effectiveness to be applied to the casting process.
With thick slab casting being used predominantly, the introduction of thin slab casting allowed for a range of process efficiencies including the reduction of total production line from 800m to 250m.

The steel produced by both BOF and EAF typically follow similar routes after the molten steel is poured from the furnace. The molten steel is transferred to the ladle where the metal chemistry is adjusted to meet the final steel product specifications, which may include adding small amounts of other metal alloys.

The steel then proceeds to the continuous caster, which casts the steel into semi-finished shapes (e.g., slabs, blooms, billets, rounds, and other special sections). Steel from the continuous caster is processed in rolling mills to produce the final steel shapes that are sold by the steel mill.

In most cases, these cast shapes will be cooled and stockpiled for later introduction into the rolling mill where the final market shape will be produced. These shapes include coiled strips, rails, and other structural shapes, as well as sheets and bars. The semi-finished products may be further processed by using many different steps, such as annealing, hot forming, cold rolling, heat treating (tempering), pickling, galvanizing, coating, or painting.

Many of these steps require additional heating or reheating. The additional heating or reheating is accomplished using furnaces usually fired with natural gas. The furnaces are custom designed for the type of steel, the dimensions of the semi-finished steel pieces, and the desired temperature.

Casting developments have aimed to reduce the number of process steps involved in producing the final product. Conventional casting machines may be up to 800m in length, containing a repeating furnace, roughers and finishers. With the advent of thin slab casting the number of stages is reduced, typically reducing machine length to 250m.

Thin slab casting and direct rolling (TSDR) technologies are nowadays one of the most promising processing routes to maintain steel as a leading material in technological applications. Initially, this process was exclusively for the production of mild steels. As industrial experience and knowledge improved, a rapid expansion of the range of products took place with higher strength grades becoming an important part of the overall production. Actually, it is widely accepted as a route to produce high value grades and it can be considered as a technology which has reached a high degree of maturity.

Originally, the Thin Slab based process was developed with the primary goal of reducing the investment and production costs related to the traditional thick slab process. The first target achieved, due to design limitations of the casters in the first generation technologies, was to serve markets with limited requirements (mainly for low added value commercial applications) and with a productivity around 1 Mtpy (per casting strand) of HRC or less. The 4th-generation of Slab casters allow production with a variety of added value steel grades that could not be previously obtained by use of a thick slab caster.

In the case of thin slab casting, the steel is cast directly to slabs with a thickness between 1.2 and 2.4 in (30 and 60 mm) instead of slabs with a thickness of 4.72 to 11.8 in (120 to 300 mm).

The method involves pouring molten steel into the Tundish at the top of the slab caster, from a ladle. They are sized with a working volume of min 100 t, which will deliver the steel at a rate of one ladle every 40 minutes to the caster. The temperatures of liquid steel in the tundish as well as the steel purity and chemical composition have a significant impact on the quality of the cast product.

The liquid steel passes at a controlled rate into the caster, which is made up of a water cooled mould in which the outer surface of the steel solidifies. In general, the slabs leaving the caster are circa 70mm thick, 1000mm wide and approximately 40m long. These are then cut by the shearer to length. To enable ease of casting a hydraulic oscillator and electromagnetic brakes are fitted to control the molten liquid whilst in the mould.